Solid state image sensors are increasingly being used in a wide variety of imaging applications as low cost imaging devices. One such image sensor is a CMOS image sensor. A CMOS image sensor includes a focal plane array of pixel cells. Each cell includes a photosensor, photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a floating diffusion region, and a transistor for resetting the floating diffusion region to a predetermined charge level. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the floating diffusion region and an access transistor for controlling the readout of the cell's contents from the source follower transistor.
Accordingly, in a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing pixel cell stored charge from the floating diffusion region.
CMOS image sensors of the type discussed above are as discussed in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, assigned to Micron Technology, Inc., which describe the operation of CMOS image sensors, and the contents each of which are incorporated herein by reference.
In a CMOS image sensor having photodiodes as the photosensors, when incident light strikes the surface of a photosensor, electron/hole pairs are generated in a p-n junction of the photosensor. The generated electrons are collected in the n-type region of the photosensor. The photo charge moves from the initial charge accumulation region to the floating diffusion region or the charge may be transferred to the floating diffusion region via a transfer transistor. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower transistor.
CMOS image sensors may have difficulty transferring all of the photogenerated charge from the photosensor to the floating diffusion region. One problem with transferring charge occurs when the n-type silicon layer of the photosensor is located close to the surface; this causes electron/carrier recombination due to surface defects. There is a need to reduce this electron/carrier recombination to achieve good charge transfer to the floating diffusion region. Another obstacle hindering “complete” charge transference includes potential barriers that exist at the gate of the transfer transistor.
Additionally, known CMOS image sensors provide only approximately a fifty percent fill factor, meaning only half of the pixel cell is utilized in converting light to charge carriers. As shown in FIG. 1, only a small portion of the pixel cell 100 is occupied by the photosensor 110 (e.g., a photodiode). The remainder of the cell100 includes the floating diffusion region 120, coupled to a transfer transistor gate 170, and source/drain regions 140 for reset, source follower, and row select transistors having respective gates 130, 150, and 160. It is desirable to increase the fill factor of the cell 100.
Image sensors may utilize a pixel cell containing a p-n-p photodiode photosensor 110 as is shown in FIG. 2, which is a cross-sectional view of the pixel cell 100 of FIG. 1, taken along line A-A′. The pixel cell 100 shown in FIG. 2 has a p-type substrate 235 with a p-well 225 formed therein. In the illustrated example, a p-type region 205 of photosensor 110 is located closest to the surface of substrate 235 and an n-type region 215 is buried beneath the p-type region 205. The p-n-p photodiode photosensor 110 has some drawbacks. First, there can be a lag problem since the pixel cell 100 uses a transfer transistor gate 170 for transferring charge to the floating diffusion region 120. Lag occurs because during integration the electron carriers are collected in the sandwiched n-type region 215 and then transferred to the floating diffusion region 120 through the transfer transistor gate 170. In order to fully utilize the generated electron carrier, it is necessary to eliminate two energy barriers to reach the floating diffusion region 120 (i.e., there is one barrier between the photosensor 110 and the transfer transistor gate 170 and another barrier between the transfer transistor gate 110 and floating diffusion region 120).
Charge leakage is another problem associated with the conventional p-n-p photodiode photosensor 110. One source of such leakage occurs when the transfer transistor gate 170 length is too short, causing sub-threshold current to become significantly high due to charge breakdown between n-type regions on both sides of the transfer transistor gate channel.
Additionally, as the total area of pixel cells continues to decrease (due to desired scaling), it becomes increasingly important to create high sensitivity photosensors 110 that utilize a minimum amount of surface area. Raised photosensors 110′, as shown in FIG. 2A, have been proposed as a way to increase the fill factor and optimize the sensitivity of a CMOS pixel cell 100′ by increasing the sensing area of the cell 100′ without increasing the surface area of the substrate 235. Further, the raised photosensor 110′ increases the quantum efficiency of the cell 100′ by bringing the sensing region closer to the microlens (not shown) used to focus light on the photosensor 110′. However, raised photosensors 110′ also have problems with leakage current across their elevated p-n junctions. Accordingly, a raised photosensor 110′ that reduces this leakage, while increasing the quantum efficiency of the pixel cell 100′, is desired.
Moreover, referring to FIGS. 2 and 2A, in CMOS image sensors, electrons are generated by light incident on the photosensor 110, 110′ and are stored in the n-type region 215, 215′. These charges are transferred to the floating diffusion region 120 by the transfer transistor gate 170 when the transfer transistor gate 170 is activated. The source follower transistor (FIG. 1) produces an output signal based on the transferred charges. A maximum output signal is proportional to the number of electrons extracted from the photosensor 110, 110′. However, a certain amount of incident light is not absorbed by the photosensor 110, 110′, but, is instead reflected from its surface and lost. The loss of this incident light decreases responsivity, dynamic range and quantum efficiency of the image sensor.
Accordingly, it is desirable to have a raised photosensor that better captures reflected incident light and directs the reflected light to the photosensor so that more of the light is absorbed and detected.